1. Field of the Invention
The present invention relates to protective shielding for instruments for the measurement of the temperature of liquids, more particularly to a refractory thermocouple shield assembly.
2. Description of the Prior Art
In various industries, such as the metal-lurgical industry, measurement of the temperature of a liquid at high temperature is required. An example is molten aluminium contained in reverberatory melting and holding furnaces. Such furnaces (hereinafter referred to as aluminium treatment furnaces) contain molten aluminium at a temperature of about 700-800° C. Such a metal-production process. is controlled by measurement of temperature. Measurements are effected by inserting into the molten metal a thermocouple either from the side of the furnace or from the top. The thermocouples used in these applications are made of metallic thermoelectric elements having a hot junction at one end. In the present state of the art, these elements are protected by use of concentric tubular inner and outer shields. The lifetime of such assemblies is essentially determined by the integrity of the outer shield during operation.
Traditionally, cast-iron outer protective shields have been used in the thermocouple assemblies of aluminium treatment furnaces. The lifetime of such assemblies is currently approximately between 3 and 8 days when used on a continuous basis. However cast iron presents several disadvantages, two of which are that cast iron is soluble in molten aluminium, and very ductile at the operating temperature of aluminium treatment furnaces. The ductility promotes plastic deformation of the shield during operation and can consequently result in breakage of the thermocouple elements.
During the last decade, ceramic protective shields made from technical ceramics have also been used. One of the better performing materials for this application is SiAlON, which is a manufactured technical ceramic comprising a solid solution of Si3N4 and Al2O3. While the lifetime of thermocouple assemblies comprising protective shields fabricated from technical ceramics can reach about one year, the brittleness of those ceramics renders it very difficult in an industrial environment to handle such assemblies without breakage for such a period of time. Moreover, technical ceramics are very expensive. This is due to the costs of the process used to produce the powder required for their manufacture, such as the vapor phase process together with the forming techniques involved, e.g., reaction bonding, use of a high-temperature isostatic press, etc.
More recently, ceramic protective shields made from castable refractories have also been tested. The maximum lifetime obtained from such protective shields is usually less than two weeks. Moreover, a drawback of castable refractories is that they are heterogeneous materials which generally have coarse grains with a diameter that may exceed 5 mm. This usually prevents the production of refractory pieces such as tubes having a low thickness, since the minimum thickness should usually be at least equal to four times the maximum grain size.
Also, the relatively high porosity of castable refractories as compared with technical ceramics confers on them relative low strength and thermal conductivity. Low thermal conductivity is detrimental to thermal shock resistance, especially when a refractory component is not sufficiently thin. Under such conditions, high thermal gradients are developed in the component during service, which promotes high induced stresses in the material. Low strength is also detrimental to thermal shock resistance when the firing temperature is not high enough to promote ceramic bonding. In such a case, thermal shock damage proceeds by long crack initiation, which is promoted by low strength. Conversely, castables fired at a high temperature develop ceramic bonds and, in such a case, thermal shock damage proceeds by short crack propagation which is promoted by-high strength. Thermal shock effects are particularly pronounced when the length of the refractory components exceeds one meter. The critical firing temperature for the interaction of the foregoing two opposing behaviors is, for aluminosilicate castables, about 1200° C. At higher temperatures, mullitisation of the castable's matrix takes place and ensures ceramic bonding within the material.
A current practice to attempt to compensate for the above-mentioned weaknesses of the castable refractories is to embody steel structures or reinforcement into their fabrication. One example is the casting of a protective layer of refractory concrete on the interior or exterior of a steel pipe or tube. The steel reinforcement increases bending resistance and prevents, catastrophic failure in the event of cracking of the refractory concrete. However, such a steel reinforcement introduces new problems. Firstly, the refractory concrete must be adequately anchored to the steel reinforcement. Secondly, the difference in thermal expansions between steel and refractory concrete usually leads to increased cracking of the concrete. Whilst steel reinforcement is feasible in some applications, it is generally impracticable for long thin-walled thermocouple shields, where the difference in thermal expansions leads to unacceptable cracking of the refractory.
Although thermal shock resistance is of great concern for thermocouple ceramic shields (technical ceramics and refractories), the major problem encountered with refractory castable tubes in aluminium treatment furnaces is their low resistance to thermal stresses induced near the metal line where the temperature gradient in the furnace is at a maximum. Even after thermal equilibrium is reached, the strength of such tubes is not high enough to enable long term resistance to stresses induced at that location. Increasing the firing temperature of the tube can be beneficial, but this increases production costs. Moreover, refractories used under cyclic conditions suffer damage by thermal fatigue. This means that, even with higher strength, such materials will have a limited service life because they have to be removed from the molten metal and then re-immersed after each campaign.
One other important limitation of standard refractory shields is that they usually retard the time response of the thermocouple assembly, especially when their thickness is high and their thermal productivity low.
Accordingly, there is a need for a thermocouple shield assembly which has increased durability in a thermal gradient environment while offering reduced time response at a relatively low cost.